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Abstract

This is a method of testing of silicon wafers by means of thermoreflectance measurements wherein an energy beam, preferably in the form of a laser beam, is used as a pump to energize specific points on a sample under test (e.g., a semiconductor wafer), causing low levels of heating of the sample at the point where the energy beam is directed, which is at such a low level of energy that it is nondestructive. Then an additional wave comprising a probe beam is directed at the same point on the sample and that beam is monitored to determine the energy level of the sample under test. The reflectivity of silicon, and most other solids, is a known function of its temperature. This effect is known as thermoreflectance.

Country

United States

Language

English (United States)

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This is a method of testing of silicon wafers by means of
thermoreflectance measurements wherein an energy beam, preferably in
the form of a laser beam, is used as a pump to energize specific
points on a sample under test (e.g., a semiconductor wafer), causing
low levels of heating of the sample at the point where the energy
beam is directed, which is at such a low level of energy that it is
nondestructive. Then an additional wave comprising a probe beam is
directed at the same point on the sample and that beam is monitored
to determine the energy level of the sample under test. The
reflectivity of silicon, and most other solids, is a known function
of its temperature. This effect is known as thermoreflectance.

Thus, by changing the temperature, the

(Image Omitted)

percentage of the probe beam, which is reflected from the
surface of the silicon, is a function of the temperature beneath the
pump beam. A parallel array of pump and probe beams can be employed.
In addition, circuits can be provided to make the measurements
desired as to the response to variations in reflectivity. In VLSI
wafer processing it is highly desirable to be able to detect the
introduction of yield-degrading defects, such as contamination, slip,
scratching, or precipitation, as quickly and as closely to the source
of the problem as possible, so corrective action can minimize the
loss. Of course, concomitant testing must be non-destructive and as
non-invasive as possible. This implies no mechanical contact

(Image Omitted)

with the wafers. Testing must be rapid
(on the order of 1 wafer per minute) and have spatial resolution of
about 100 microns or better. Here, thermoreflectance, one of the
modes of modulation spectroscopy that has been used for almost 20
years to establish the band structure of semiconductors, has been
modified and adapted to the present purpose. In contrast to
conventional thermoreflectance, where Joule heating is induced by
electrical contacts made into broad areas of a perfect sample, we
induce localized and periodic heating by optical excitation. In
further contrast to conventional thermoreflectance, where one varies
the probe wavelength to obtain a spectrum from which band structure
information is obtained, we, having established the band structure of
Si and other materials of interest, keep the probe wavelength fixed
and measure the variation of the amplitude of the reflectance
transient, which is proportional to the local variation of surface
temperature, as a function of the location on the wafer. The degree
to which each region of the sample heats is affected by processes
which are strong functions of the degree of perfection of the wafer
in the region excited. These include: i) the rate of nonradiative
recombination of the photoexcited carriers; ii) the rate of diffusion

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of these carriers; iii) the optical absorption constant at the
excitation, or "...